Turbine blades in high performance gas turbine engines, particularly in those engines used to power aircraft, need to be able to withstand high temperatures. These temperatures may be higher than the melting point of the material of the blade, and this situation is addressed by cooling the surface of the blade by means of a stream of cool air emanating from an interior cavity of the blade through carefully designed and spaced apertures communicating with the surface.
The interior cavity is usually made by casting the blade round a ceramic core. Such a ceramic core has a silica base formulation which is sintered at approximately 1100.degree. C. and is designed to give good hot strength for dimensional control. The silica base of the ceramic also enables the core subsequently to be leached from the casting by means of a caustic alkali solution.
However, a problem arises when such cores are used in the casting of directionally solidified or single crystal alloys. These alloys are typical of the new generation of nickel-based superalloys now used in the manufacture of high performance blades. The problem is that the casting temperature is about 400.degree. C. higher than the sintering temperature, and the core is accordingly subject during casting to a substantially higher temperature than that at which it was sintered. The core is therefore liable to distortion during the temperature changes of the casting process.
In the present established casting process a ceramic core is positioned within a blade shaped die by means of chaplets made of wax or other low melting point material. These chaplets extend from the core to the interior surface of the die. Wax is than injected into the cavity between the die and the core and is allowed to solidify. When the wax has solidified the die is removed and further chaplets comprising cylindrical platinum pins are posted through the wax to the surface of the core. The wax is then invested with a suitable thickness of ceramic slurry to provide a ceramic mold. The platinum chaplet pins extend from the surface of the core into the investment mold.
After the requisite thickness of investment coating has been applied the mold is heated so as to fire the investment coating and melt out the wax together with the original wax chaplets. The core is therefore now supported within the investment mold by the platinum chaplet pins which extend from the core into the mold. The casting of the alloy into the cavity between the investment mold and the core is then done. The mold is subsequently removed when the alloy has solidified.
It will be appreciated that there is a major disadvantage in that the platinum chaplet pins extend to the surface of the resulting casting, rendering it necessary to carry out extensive dressing of the casting to remove the projecting portions of the pins and correct the external profile. The portions of the platinum pins within the casting will have dissolved in the superalloy. In practice, platinum is not known to adversely affect the properties of the superalloy.
At a convenient stage in the process the ceramic core is leached out of the casting in the manner indicated above, to provide the required internal cavity.
A process which avoids the surface dressing requirements of the established prior art process is disclosed in UK patent GB 2118078. In this process the metal pins are replaced by chaplets which abut the mold cavity surface. The chaplets used are dimensioned in accordance with the desired wall thickness of the article to be cast, and configured in accordance with line or surface abutment at the mold cavity interface. Because the chaplets are contained entirely within the mold cavity no surface extensions are produced.
From a review of this prior art it will be appreciated, however, that the chaplets proposed are not entirely suitable for casting directionally solidified or single crystal alloys. When casting these alloys it is necessary to minimise the contact area between the chaplets and the mold cavity surface. If this area is too great grain growth may initiate at the abutment interface and disrupt the desired grain growth pattern. There is a tendency for this to occur when surface contact and line contact chaplets are used.
It is an object of the present invention to overcome the above disadvantages.
According to the present invention there is provided in an investment casting process for casting a component having at least one internal cavity, the process comprising the known steps of providing at least one core member corresponding to the shape and size of the cavity, positioning the core member in a removable wax impression die, providing at least one support member for the core, the support member being dimensioned in accordance with a desired wall dimension of the component to be cast, positioning the support member on the surface of the core, injecting a molten wax into the die, removing the wax once the die has solidified, investing the wax in ceramic, firing the ceramic and removing the wax to provide a ceramic mold, and casting a molten material into the mold, the improvement comprising, adapting the suport member for point contact abutment, and positioning the support member on the surface of the core to locate the core in spaced apart relation to an adjacent mold surface by point contact abutment with the adjacent mold surface.
It will be understood that "wax" in the context of this specification means not only esters of fatty acids or mixtures of relatively high melting point hydrocarbons that are commonly used in wax casting processes but other, easily melted, polymers that are solid at room temperature and which fulfil the same purpose as a casting wax.
It will be seen, therefore, that the present invention provides a cast component that has no externally projecting chaplets pins because the spacing member, which is in effect a chaplet pin, never extends beyond the external surface of the casting defined by the inner surface of the mold. Further, the spacing member, when it dissolves in the casting, leaves no holes.